Root Rot Management in Common Bean (Phaseolus vulgaris L.) Through Integrated Biocontrol Strategies using Metabolites from Trichoderma harzianum, Serratia marcescens, and Vermicompost Tea

Abstract

Common bean (Phaseolus vulgaris L.) is an essential food staple and source of income for small-holder farmers across Africa. However, yields are greatly threatened by fungal diseases like root rot induced by Rhizoctonia solani. This study aimed to evaluate an integrated approach utilizing vermicompost tea (VCT) and antagonistic microbes for effective and sustainable management of R. solani root rot in common beans. Fourteen fungal strains were first isolated from infected common bean plants collected across three Egyptian governorates, with R. solani being the most virulent isolate with 50% dominance. Subsequently, the antagonistic potential of vermicompost tea (VCT), Serratia sp., and Trichoderma sp. was assessed against this destructive pathogen. Combinations of 10% VCT and the biocontrol agent isolates displayed potent inhibition of R. solani growth in vitro, prompting in planta testing. Under greenhouse conditions, integrated applications of 5 or 10% VCT with Serratia marcescens, Trichoderma harzianum, or effective microorganisms (EM1) afforded up to 95% protection against pre- and post-emergence damping-off induced by R. solani in common bean cv. Giza 6. Similarly, under field conditions, combining VCT with EM1 (VCT + EM1) or Trichoderma harzianum (VCT + Trichoderma harzianum) substantially suppressed disease severity by 65.6% and 64.34%, respectively, relative to untreated plants. These treatments also elicited defense enzyme activity and distinctly improved growth parameters including 136.68% and 132.49% increases in pod weight per plant over control plants. GC-MS profiling of Trichoderma harzianum, Serratia marcescens, and vermicompost tea (VCT) extracts revealed unique compounds dominated by cyclic pregnane, fatty acid methyl esters, linoleic acid derivatives, and free fatty acids like oleic, palmitic, and stearic acids with confirmed biocontrol and plant growth-promoting activities. The results verify VCT-mediated delivery of synergistic microbial consortia as a sustainable platform for integrated management of debilitating soil-borne diseases, enhancing productivity and incomes for smallholder bean farmers through regeneration of soil health. Further large-scale validation can pave the adoption of this climate-resilient approach for securing food and nutrition security.

Keywords: Biological control; Common bean; Integrated disease management; Root rot; Vermicompost tea.

Fig. 1

Typical natural symptoms of common bean root rot on common bean plant behind healthy one. a: Pure culture of R. solani isolate grew on PDA medium. b: The black microsclerotia are visible on the root and the lower part of the stem of the diseased plant. c: Healthy common bean plant (control)

Fig. 2

The abundance of isolated fungi (%) from common bean plants infected with damping off and root rot

Fig. 3

Pathogenicity of fungal isolates from infected common bean cv. Giza 6 plants in terms of pre- and post-emergence damping-off percentage. a: Rhizoctonia sp. isolates, b:Fusarium sp. isolates, c: Macrophomina sp. isolates

Fig. 4

In vitro antagonistic activity of a: Serratia S1 isolate, b: Trichoderma T15 isolate, and c:VCT extract against R. solani after incubation at 25 ± 2℃ for 5–7 days expressed as IZD (cm)

Fig. 5

Colonies of the isolated bacterial and fungal isolates. a: R4 fungal isolate and b: microscopic examination of R4 isolates indicating the hyphae branch at right angles or acute angles. c: S1 bacterial isolate and d: microscopic examination of S1 isolate indicating Gram-negative short rods. e: T15 fungal isolate indicating greenish colonies with branching or simple conidiophores in brush-like cluster. f: Microscopic examination of T15 isolate indicating green or yellow-green in color conidiospores and are borne in clusters at the tips of the conidiophores

Fig. 6

a: Neighbor-joining tree based on 16S rRNA sequences obtained from BLAST search indicating the position of Serratia marcescens isolate and related strain. b: Neighbor-joining tree based on 18S rRNA sequences obtained from BLAST search indicating the position of Rhizoctonia solani and Trichoderma harzianum isolates and related strain

Fig. 7

Effect of single and different combinations of treatments on root rot disease severity on common bean cv. Giza 6 under field conditions

Fig. 8

Effect of single and different combinations of treatments on common bean cv. Giza 6 pod weight/plant under field conditions

Fig. 9

a: Effect of single and different combinations of treatments on shoot length and root length of common bean cv. Giza 6 plants under field conditions. (a) VCT + T. harzianum, (b) VCT + EM, (c) VCT + Serratia marcescens, (d) EM1. b: Effect of vermicompost on common bean plant. The first image (a) appears to show vermicompost, which is a nutrient-rich soil amendment produced by the breakdown of organic matter by worms or other decomposer organisms before soaking treatment. The second image (b) depicts a healthy, lush potted plant, likely benefiting from compost amendment, vermicompost

Fig. 10

Effect of single and different combinations of treatments on root weight, shoot weight, and dry weight of common bean cv. Giza 6 plants under field conditions

Fig. 11

Peroxidase (POD) and polyphenol oxidase activities, expressed as a rate of hydrogen peroxide conversion per fresh mass (FM) in common bean cv. Giza 6 leaves as influenced by different treatments